Calcium nitrate based fertilizer

ABSTRACT

Homogeneous calcium nitrate based fertilizers containing sulphur, and method for their preparations. Solid gypsum and/or a slow reacting sulphate mineral is mixed with melted calcium nitrate fertilizer at a temperature of 90-110° C. and kept at this temperature for 3-15 minutes and subsequently particulated at a temperature of 80-110° C. The particulation by prilling is preformed at 95-110 ° C. and the particulation by granulation is performed at 90-110° C. The fertilizer is a homogeneous composition of a calcium nitrate fertilizer and gypsum and/or a slow reacting sulphate mineral comprising 0.1-5.0 weight % SO 4 —S, 14-19 weight % water soluble calcium and 16-21 weight % total calcium, and 0-3.0 weight % water soluble magnesium. The fertilizer may contain microamounts of Se and/or Co. and/or micronutrients like Mn, Cu, B and Zn.

The present invention relates to calcium nitrate based fertilizerscontaining sulphur and a process for making such fertilizers.

Calcium nitrate is widely used within horticulture and as a dry producton open fields. There are two major types of calcium nitrate, one madefrom acidulation of calcium carbonate by nitric acid and one obtained asa by-product from nitrophosphate fertilizer processes. The latternormally contains 75-81 weight % Ca(NO₃)₂, 5-9 weight % NH₄NO₃ and 12-14weight % water of crystallization. This type is usually called NH—CN(Norsk Hydro Calcium Nitrate). In the following the abbreviation CN willbe used for both these types of fertilizer.

Over the last years there has been an ever increasing demand for sulphurin fertilizers in general, even for special applications ofCN-fertilizers sulphur has been required. For some markets and cropsthere has also been a demand for CN-fertilizers containing Mg, Se, Coand some micronutrients in addition to sulphur.

In order to meet the demands for new fertilizers the simplest solutionseemed to be blending various S-containing fertilizers like ammoniumsulphate or potassium sulphate, but when other ingredients like Mg, Seetc. should be added, segregation problems would inherently arise.Another criteria will of course be the price of the various componentsand it would be advantageous to use cheap sulphur sources like gypsum,kieserite and langbeinite, but again segregation problems wereexperienced.

Preparation of homogeneous CN-based fertilizers by particulation ofmelts or solutions of CN and sulphur sources like ammonium- or potassiumsulphate did however prove to be most difficult due to the low abilityto tie up water of crystallization possessed by these two sulphursources. Possible application of gypsum and/or kieserite for makinghomogeneous fertilizers was then investigated.

From Japanese patent application J54085957 it is known preparation of afertilizer comprising mixing calcium nitrate, gypsum and superphosphateof lime and then granulating the mixture. Mixing is performed at100-120° C. and the molten mixture is extruded, cut to a size of 2-3 mmand cooled and dried. The amount of superphosphate of lime is 50-70% ofthe calcium nitrate. Firstly, this fertilizer contains large amounts ofphosphate and no magnesium. Secondly, the particulation process is mostuneconomical.

Further it is known from J53027564 to coat CN-fertilizers with gypsumduring the spraying of CN melt in cooling air containing gypsum powder.This fertilizer is stated to have a prolonged effect. Its sulphurcontent will however be far too low to meet the requirements which thepresent invention has to meet.

It is also known to add minor amounts of CN to granulated ammoniumsulphate as a binder and to add minor amounts of ammonium sulphate andcalcium nitrate to ammonium nitrate to improve its crushing strength,but none of these publications solve the problems related to makinghomogeneous CN-based fertilizers.

The main object of the present invention was to arrive at a homogeneousCN-based fertilizer containing sulphur and optionally magnesium,selenium, cobalt and micronutrients.

Another object was to arrive at a cost effective continuous process formaking homogeneous sulphur containing CN-based fertilizers which couldbe particulated.

A further object was to arrive at a process for making homogeneoussulphur containing CN-based fertilizers containing magnesium andoptionally selenium, cobalt and micronutrients.

Application of normal S-sources like ammonium sulphate and potassiumsulphate were first investigated. It was then found that when thesesources are mixed with CN melt the following reactions will take place:

1. (NH₄)₂SO₄+Ca(NO₃)₂→CaSO₄+NH₄NO₃

2. K₂SO₄+Ca(NO₃)₂→KNO₃+CaSO₄

The formation of gypsum consumes calcium nitrate and the melt loses itsability to solidify due to less power to tie up water ofcrystallization. Accordingly, the two S-sources were found to be toodifficult to apply in CN-based fertilizers.

Gypsum (CaSO₄1/2 H₂O) was expected to not react with CN melt and shouldtherefore constitute a cheap S-source. Even though gypsum is not watersoluble, several agronomical examinations have shown that gypsum isreadily available to plants. Different gypsum sources can be utilized,such as dead burned gypsum and gypsum with hydrate water up todihydrate, the hemi hydrate form is however preferred.

Kieserite (MgSO₄ H₂O) is both a Mg- and S-source which is considered tobe water soluble. By mixing this salt into a CN melt the followingreaction was found to take place:

3. MgSO₄H₂O+Ca(NO₃)₂→CaSO₄+Mg(NO₃)₂+H₂O

It was further found that if the Mg(NO₃)₂ content became too high,solidification problems would occur. However, trials carried outrevealed surprisingly that reaction no. 3 did not take place at asignificant rate. During further laboratory tests at which kieserite wasthoroughly mixed into CN melt for several minutes it was confirmed thatsaid reaction did not result in solidification problems. Theinvestigations also showed that reaction 3 indeed did take place at thesurface of each kieserite crystal, but the gypsum formed a thin coatingthat protected the crystals from further attack by the CN melt. Thisformation of gypsum coating and the slow dissolving time of thekieserite mineral in general seemed to give possibilities forhomogeneous distribution of fine kieserite crystals in the CN melt priorto solidification and thereby make it possible to utilize this formaking a homogeneous CN-based fertilizer containing both sulphur andmagnesium.

Based on the results from the above investigations, several tests wereperformed in a pilot plant. It was then confirmed that by using gypsumand slow reacting sulphate minerals like kieserite, langbeinite etc.together with a CN melt it would be possible to make homogeneousS-containing fertilizers which met the above stated objects of theinvention.

Thus, according to the invention, solid gypsum and/or a slow reactingsulphate mineral could be mixed with melted calcium nitrate fertilizerat a temperature of 90-110° C. and kept at this temperature for 3-15minutes and subsequently particulated at a temperature of 80-110° C.

The particulation by prilling should be performed at 95-110° C. and thegranulation at 90-110° C.

Microamounts of Co and/or Se and/or micronutrients like Mn, Cu, B, andZn could be added as solutions of their salts to the mixture prior toparticulation.

One preferred method comprises mixing 4-23 weight % gypsum, based on thetotal composition, with a melted calcium nitrate fertilizer at 103-106°C. during stirring and keeping the mixture at this temperature for 5-10minutes and then particulate the thus formed slurry.

Another preferred method comprises mixing 4-23 weight % kieserite duringstirring with a melted calcium nitrate fertilizer for 6-12 minutes andsubsequently particulate the thus formed slurry.

Fertilizers according to the invention comprise a homogeneouscomposition of a calcium nitrate fertilizer and gypsum and/or a slowreacting sulphate mineral comprising 0.1-5.0 weight % SO₄—S, 14-19weight % water soluble calcium and 16-21 weight % total calcium and0-3.0 weight % water soluble magnesium. Preferably said new fertilizercontains 1.0-3.0 weight % SO₄—S. The preferred content of magnesium is0.1-4.0 weight %.

The scope and special features of the invention are as stated in theattached claims.

The invention is further explained by the following examples and thecomments to these examples:

EXAMPLE 1

This example relates to formation of prilled homogeneous CN-basedfertilizer by mixing 12 weight % kieserite crystals into NH—CN melt at105° C. during stirring at this temperature for about 10 minutes andthen spraying the slurry through a nozzle. The formed dropletssolidified as they fell through cooling air.

The product thus formed had a homogeneous composition and had thefollowing analysis:

15.0% N

2.6% SO₄—S

2.0% Mg (water soluble)

16.8% Ca (water soluble)

EXAMPLE 2

This example shows drum granulation of a homogeneous CN-basedfertilizer. 7 weight % kieserite crystals were mixed into 93 weight %NH—CN melt at 105° C. and stirred for about 6 minutes. The slurry wasthen sprayed into a drum containing fines from example 1. Hardhomogeneous granules were formed with the following composition:

15.2% N

1.5% NH₄—N

13.7% NO₃—N

1.5% SO₄—S

1.1% Mg (water soluble)

17.5% Ca (water soluble)

EXAMPLE 3

This example shows granulation of homogeneous CN-based fertilizercontaining sulphur. 11.5 weight % calcium sulphate hemi hydrate (22.1%S) was mixed with 88.5 weight % NH—CN melt at about 105° C. and stirredfor about 5 minutes and subsequently sprayed into a rotating drumcontaining NH—CN fines.

The thereby formed product had the following analysis:

15.0% N

2.5% SO₄—S

16.8% Ca (water soluble)

20.0% total Ca

EXAMPLE 4

This example shows preparation of homogeneous CN-based fertilizercontaining sulphur and magnesium and in addition selenium. 7 weight %kieserite and some droplets of a 25 weight % Na₂SeO₄ solution were mixedinto a NH—CN melt at about 105° C. and stirred for about 5 minutes. Themelt was solidified as in example 2.

The formed product had the following analysis:

15.2% N

1.5% NH₄—N

13.7% NO₃—N

1.5% SO₄—S

1.1% Mg (water soluble)

17.5% Ca (water soluble)

25 ppm Se

Cobalt can be incorporated in the same way as selenium by addition ofdroplets of a COSO₄ solution giving 0.02% of Co.

EXAMPLE 5

This example shows preparation of homogeneous CN-based fertilizercontaining both sulphur and magnesium. 4 weight % gypsum (calciumsulphate hemi hydrate) and 6 weight % kieserite were thoroughly mixedinto 90 weight % of NH—CN melt at about 105° C. for about 4 minutes. Thethereby formed slurry was then sprayed into a granulating drumcontaining fines. Hard. homogeneous granules were formed with thefollowing composition:

15.1% N

2.2% SO₄—S

1.0% Mg

16.7% water soluble Ca

17.8% total Ca

The main advantages attained by using both gypsum and kieserite formaking such fertilizers are that gypsum is a cheaper sulphur source thankieserite and that the product is somewhat less hygroscopic than aproduct using only kieserite as sulphur and magnesium source.

From the above examples it can be seen that it is possible to preparehomogeneous S-containing fertilizers based on CN-fertilizers and aS-source. The new fertilizer can be particulated, for instance byprilling or granulation, both pan granulation and drum granulation.Further tests showed that a rather wide range of such fertilizers couldbe prepared. It was then found that the various components should bewithin the following ranges:

SO₄—S 0.1-5.0 weight % preferably: 1.0-3.0 weight % Mg (w.s) 0-4.0weight % preferably: 0-2.3 Ca (w.s) 14-19 weight % preferably: 16-19 Ca(total) 16-21 weight %

In addition fertilizers of the above type can contain microamounts ofcobalt and selenium, and the usual fertilizer micronutrients likemanganese, copper zinc, boron etc.

Preparations of the above new fertilizers can be performed at thefollowing conditions:

Recommended mixing temperature: 90-110° C.

Recommended retention time :3-15 minutes

Recommended particulation temperatures

Prilling: 100-110° C.

Granulation: 80-100° C.

The products of examples 1 and 2 were bagged and stored at a pressure of1 kg/cm² for three months and there were no signs of caking. Also therate of water absorption was tested for these new fertilizers andcompared with conventional CN-fertilizers. The rate of absorption wasmeasured at 60% relative humidity and 25° C. The recorded values arelisted in the below table:

TABLE 1 Water absorbed % Water absorbed % Product 1 hour 24 hours NH—CNuncoated 2.0 28.1 NH—CN coated 0.4 7.0 NH—CN + 1.5% S 2.7 9.7 (uncoatedkieserite) NH—CN + 1.5% S 0.7 32 (coated kieserite) NH—CN + 2.5% S 2.933 NH—CN + 2.5% S 0.5 7.5 (coated gypsum) NH—CN + 15% S + Se 0.6 31.5(coated)

From these data it can be seen that fertilizers based on CN andkieserite had somewhat higher hygroscopicity compared to pureCN-fertilizer. The increase is very modest and due to the formation ofsome Mg(NO₃)₂. The increase of water absorption can be compensated byincreasing the amount of coating applied.

By the present invention the inventors have managed to arrive at amethod for making a new type of fertilizer being a homogeneousfertilizer based on calcium nitrate and containing the required amountof sulphur. Said new fertilizer may also contain magnesium andmicroamounts of Se and Co and micronutrients like Mn, Cu, B, Zn etc.

What is claimed is:
 1. A method for making calcium nitrate basedfertilizers containing sulphur, which consists of mixing solid gypsumand/or a slow reacting sulphate mineral with melted calcium nitratefertilizer at a temperature of 90-110° C., maintaining the mixture atthis temperature for 3-15 minutes, and subsequently particulating themixture at a temperature of 80-110° C.
 2. A method according to claim 1,characterized in that 4-23 weight % gypsum, based on the totalcomposition, is mixed with the melted calcium nitrate fertilizer at103-106° C. while stirring and kept at this temperature for 5-10 minutesto form a slurry and the formed slurry is particulated by prilling orgranulation.
 3. A method according to claim 1, characterized in that4-23 weight % solid kieserite is mixed while stirring with the meltedcalcium nitrate fertilizer for 6-12 minutes to form a slurry and theformed slurry is subsequently particulated.
 4. A method according toclaim 1, characterized in that: the particulation is by prillingperformed at 95-110° C. or by granulation performed at 90-110° C.
 5. Amethod according to claim 1, characterized in that: selenium, cobalt,and micronutrients are added as solutions of their salts into themixture prior to particulation.
 6. A method according to claim 5,wherein the micronutrients comprise manganese.
 7. A calcium nitratebased fertilizer containing sulphur, characterized in that thefertilizer is a homogeneous composition of a calcium nitrate fertilizerand gypsum and/or a slow reacting sulphate mineral consisting of 0.1-5.0weight % S as derived from SO_(4,)14-19 weight % water soluble calciumand 16-21 weight % total calcium, and 0-4.0 weight % water solublemagnesium.
 8. A fertilizer according to claim 7, characterized in thatthe calcium based fertilizer containing sulfur contains 1.0-3.0 weight %S as derived from SO₄.
 9. A fertilizer according to claim 7,characterized in that: the calcium nitrate based fertilizer containingcontains sulphur 0-2.3 weight % water soluble magnesium.
 10. Afertilizer according to claim 7, characterized in that: the calciumnitrate based fertilizer comataining sulphur contains microamounts of Seand/or Co, and/or micronutrients.
 11. A fertilizer according to claim10, wherein the micronutrients are selected from the group consisting ofMn, Cu, B and Zn.